The active materials for a positive electrode of an electric cell of high performance are typically required to have a high electromotive force, a high open circuit voltage and, in addition, a small overvoltage on discharge, a good flat characteristic on the discharge curve and a large discharge capacity per unit weight when they are used in combination with an appropriate negative electrode. Furthermore, the active materials are required to be neither decomposed nor dissolved in the electrolyte of an electric cell and they also have to be stable for a long period of time.
Fluorinated carbon materials having the general chemical formula (CFx)n, where x is a number between 0 and 2 and n is an indefinite number greater than 2, have been known to be useful as cathode materials for lithium batteries and also have been used in lubricating applications. (CFx)n hereinafter is abbreviated as CFx for purposes of the present specification. Fluorinated carbon is prepared by the reaction of fluorine gas with many of the various forms of carbon, including graphite, petroleum coke, coal coke, carbon black, or carbon fiber. The reaction between fluorine and carbon is carried out at temperatures ranging from 250° C. to 600° C. and the reaction time is usually in the range of 1 to 24 hours.
It has been know for quite some time that fluorinated carbon compounds can be used as active cathode materials in non-aqueous batteries. Particular interest has centered on systems employing this cathode material, a non-aqueous electrolyte and a highly active metal anode such as lithium or sodium. As an example of such a system, Braeuer et al. have disclosed in U.S. Pat. No. 3,514,337 a high energy density battery composed of CFx, where x is in the range 0.1 to 0.28. In the Watanabe, et al., U.S. Pat. No. 3,536,532 the patentees describe a high energy density battery utilizing CFx where x falls in the range of 0.5 to a maximum of 1. The fluorinated carbon cathode material prepared from crystalline carbon (e.g., graphite), exhibited higher energy densities and improved discharge performance when compared to the material described by Braeuer. An additional disclosure by Watanabe, et al., in U.S. Pat. No. 3,700,502 describes a high energy density system employing fluorinate carbon of the type CFx wherein x is in the range of greater than 0 and up to 1 and is prepared from carbon sources such as charcoal, activated carbon or coke. These batteries exhibited extended shelf life due to the stability of the fluorinated carbon in the electrolyte. In both of these systems, the electrolyte was a non-aqueous solution of an organic solvent (i.e. propylene carbonate and the like) and lithium perchlorate. The anode active material is an alkali metal such as lithium or sodium.
Another battery, which is composed, of fluorinated carbon having an x value of greater than 1 and up to and including 2 is described in Gunther U.S. Pat. No. 3,892,590. The materials of that patent are described as exhibiting higher energy densities than the prior art due to the increased fluorine content.
In U.S. Pat. No. 4,271,242, Toyoguchi, et. al. disclose the use of fluorinated carbons obtained by fluorinating carbon having a lattice constant of 3.40-3.50 A in its (002) plane. The carbon is selected from among petroleum cokes and coal cokes and the resulting battery has excellent discharge and shelf life characteristics. The fluorinated carbon materials described in this patent are generally accepted as the industry standard for lithium batteries employing fluorinated carbon cathodes and such materials are widely used in commercial battery production.
In the Russian Journal of Electrochemistry Vol. 36 No. 12, 2000, p 1325 Zhorin and Smirnov evaluated the performance of several types of fluorinated carbons including fluorinated carbon black, fluorinated coke and fluorinated carbon fiber. Their results show that fluorinated coke is superior to fluorinated carbon fiber. But these authors did not recognize the advantages of using a mixture of fluorinated carbons produced from anisotropic and isotropic carbon, where the anisotropic material is carbon fiber and the isotropic material is graphite.
Additionally, it is known that a wide range of implantable electronic devices are provided for surgical implantation into humans or animals. One common example is the cardiac pacemaker. Other examples of implantable devices include devices for stimulating or sensing portions of the brain, spinal cord, muscles, bones, nerves, glands or other body organs or tissues. Implantable devices are becoming more and more complex and commonly include sophisticated data processing hardware such as microprocessors, memory devices, or other large scale integration (LSI) devices. Often, the devices are designed for transmitting signals to remote sensing devices. With the increase in the sophistication of implantable devices and in particular with the need to reliably transmit signals to sensors external to the body, the need for improved power cells for powering the implantable devices has increased greatly. There are, of course, limitations on the design and configuration of power cells for use in implantable devices, especially with regard to the size and shape thereof. Moreover, the power cells for the implantable devices must be highly reliable and be capable of providing an adequate amount of current and voltage for an extended period of time.
The present inventors have, therefore, made extensive studies to develop active materials of a fluorine type for the positive electrode of an electric cell having high performance and, as a result, have found active materials for the positive electrode which not only almost perfectly satisfy the above described requisites for the positive-electrode active materials of an electric cell of high performance, but also exceed conventional active materials for the positive-electrode in various performances for the electric cell.
The prior art is silent regarding a cathode composition comprised of a mixture of fluorinated carbons produced from anisotropic and isotropic carbons, where the anisotropic material is carbon fiber and the isotropic material is graphite. Applicants' have discovered that such a mixture offers substantial improvements in discharge characteristics over prior art fluorinated carbon electrode compositions when used as an active cathode material in a non-aqueous battery. The prior art is also silent regarding the use of batteries containing the novel fluorinated materials of the invention in biomedical applications including implantable devices.